Composition for preservation of metals, process and article



United States Patent Olfice 3,400,023 COMPOSITION FOR PRESERVATION OF METALS, PROCESS AND ARTICLE Louis McDonald, Altadena, Calif., assignor to Kelite Corporation, Los Angeles, Calif., a corporation of California No Drawing. Filed May 11, 1964, Ser. No. 366,591 22 Claims. (Cl. 148-615) ABSTRACT OFTHE DISCLOSURE In the phosphatizing of metals, the quality of the phosphatized coating is greatly improved by using a process involving a phosphatizing composition comprising an aqueous bath containing a major portion of a water soluble inorganic phosphorous compound and a minor portion of an ester of an acid of phosphorous.

This invention relates to the preservation of metals and has particular reference to compositions and processes for the protection of metals from corrosion.

Phosphoric acid compositions and compositions containing acid phosphate salts of metals such as iron, zinc, manganese and sodium have been used extensively to treat ferrous alloys, zinc, aluminum and cadmium alloys to improve their corrosion behavior both before and after application of protective and/ or decorative coatings. The process employed is known in the art as phosphatizing. In general there are two basic approaches employed to provide phosphate coating protection to certain metals. One approach is to impart a converted coating wherein the hydrogen ions of a bath are made to react with the metal surface to form acid metal salts in the solution. As the free acids present continue to attack the metal surface there then occurs a shift in the equilibrium concentration of acid metal salts. This is followed by the deposition of the acid metal salts at the surface of the metal and their conversion in situto insoluble metal salts with the liberation of hydrogen ions to solution. In the processof treating a ferrous metal surface with phosphoric acid solution or acid phosphate salt solution/the conversion coating deposited is a ferrous phosphate coating. It is characterized by the following properties: (1) Low density.

(2) Low weight in terms of mg./ sq. ft. of surface.

(3) Relatively poor protection 'against'the influence of galvanic current.

(4) High porosity.

(5) Fine grain structure, sometimes amorphous.

(6) Good ductility.

(7) Relatively low cost.

(8) Relatively non-adherent.

(9) Deposits on the basis metal at a relatively low rate.

The second approach provides'a coating that can be defined as a combination converted-deposited coating. It is usually comprised of zinc and/or manganese phosphate salts and is deposited from a solution of a controlled level of free acidity and a controlled level of concentration of acid metal saltssuch as zinc dihydrogen phosphate. In the operation of this type of bath a shift in the concentration of acid metal salts occurs readily when the free acid present attacks the metal surface and a deposition of zinc phosphate on the metal surface occurs. Theconverted deposited type of coating, e.g., zinc phosphate, is characterized by the following properties:

1 Low density. (2) High weight in terms of mg./sq. ft. of surface.

(3) Relatively good protection against the influence of galvanic currents.

(4) Crystalline structure.

(5) High porosity.

(6) Poor ductility, depending upon crystal size.

(7) Relatively high cost.

(8) Relatively good adherence.

(9) Relatively high rate of deposition.

It is well known that the converted-deposited type of coatings, especially zinc and manganese phosphate coatings are superior in respect to the protection they afford, the speed at which they can be deposited, and their ability to bond paint. This is due to the higher weight of coatings imparted from the bath and to the chemical nature of the coatings. While affording somewhat better protection than the ferrous phosphates, they are more :costly, more difiicult to control from the standpoint of bath operation and require greater thickness of organic decorative and/or protective coating than does a ferrous phosphate coating to obtain a finish of comparable levels of gloss 'and smoothness.

A primary object of the present invention is to provide improved converted coating and converted-deposited coating compositions and processes for use of same. Other objects and advantages of this invention it is believed will be readily apparent from the following detailed description of preferred embodiments thereof.

It has now been discovered that the qualities in which ferrous phosphate coatings are deficient, relative to zinc phosphate coatings, can be enhanced by the use of certain additives in an aqueous phosphatizing bath or in an aqueous bath used to clean the metal prior to phosphatizing. These organic additives are also capable of enhancing the quality of converted-deposited coatings such as zinc phosphate coatings, but their effect is less pronounced in comparison with their effect on the converted type of coating such as ferrous phosphate coating. This singular improvement in quality of converted and converted-deposited coatings is achieved through the use of organic additives which are characterized by the property of being moderately to difficultly soluble in water and at the same time having the property of behaving as a solvent for acids of phosphorus and less effectively as a solvent for salts of acids of phosphorus. Organic compounds that exhibit these solubility properties comprise the esters of acids of phosphorus, more particularly the esters of phoshporic acid, phosphorous acid, phosphonic acid and the complex acid phosphates. The preferred compounds are the trialkyl esters of phosphoric acid as exemplified by the compounds tripropyl phosphate, tributyl phosphate, triamyl phosphate, trihexyl phosphate and trioctyl phosphate. Lower molecular weight phosphoric esters such as trimethyl or triethyl phosphate are less desirable due to their moderate water solubility and their lack of stability in aqueous solution. Higher molecular weight phosphate esters are less preferred from an economic standpoint, their cost in use being proportional to their molecular Weights. Acid phosphorous esters and their salts are less desirable because of moderate water solubility and ability to hydrolyze in aqueous solution. Aryl and alkylaryl phosphoric esters such as triphenyl phosphate and tricresyl phosphate are less preferred because of their relatively poor behavior as solvents for phosphoric acid as compared to the lower molecular Weight alkyl phosphoric acid esters. Additionally, the aryl and alkylaryl phosphoric esters are toxic and therefore less desirable. Alkyl esters of complex phosphates such as tetraethyl pyrophosphate are effective but less desirable to use because of their higher order of toxicity. Phosphonic acid esters are useful but due to their relatively high molecular weight and cost per pound mol relative to lower molecular weight trialkyl phosphoric esters and their lower order of solvent behavior for acids of phosphorus they constitute a less preferred species. Thus, the most preferred esters of acids of phosphorus are the tripropyl and tributyl phosphates.

The surprising and unexpected influence of the esters of acids of phosphorus on converted and converteddeposited coatings is believed to reside, at least in part, in the ability of the esters to orient as a mo] dimensional layer at the solid-liquid interface of the metal and the phosphatizing bath, and to their ability to behave as solvents for acids of phosphorus. In the presence of an ester of an acid of phosphorus at the metal-liquid interface their is little evidence of polarization or gassing when the acids of phosphorus or the acid salts of acids of phosphorus react with the metal. Thus, as the metal is converted to phosphate or as metal phosphates are deposited from solution the converted or converted-deposited coating is applied in a depolarized environment. Accordingly, the phosphate coating is imparted as a dense, nonporous film of fine grain structure. Additionally, the coating deposits at an accelerated rate as compared to the coating from a conventional aqueous phosphate coating bath. This apparently occurs because a layer or film of the ester of the acid of phosphorus acts as a solvent medium and a diffusion layer for acids of phosphorus and additionally behaves as a solvent medium of a narrower range of solubility for acid metal phosphate salts as compared to an aqueous medium. Therefore, shifts in equilibrium solubility occur rapidly with slight increases in the amount of solute brought into solution in the oriented diffusion layer. Thus, for the reason that esters of acids of phosphorus show stronger solvency for acids of phosphorus than for salts of acids of phosphorus, salts of acids of phosphorus are deposited rapidly as the free acids or acid salts react with the basis metal to form salts and acid salts respectively. When the salts of acids of phosphorus form in the presence of the preferred esters of acids of phosphorus a gel forms and deposits as a coating comprised of insoluble metal phosphate salts and organic phosphates.

The following specific examples illustrate the compositions and processes of the present invention, but the invention is not to be limited to the specific details thereof:

EXAMPLE 1 A control bath of the following phosphatizing composition was made up at a level of concentration of 1 oz./ gallon; having a pH of 3.0:

Material: Wt. percent Zinc dihydrogen phosphate 1.70 Phosphoric acid (75%) 24.20 Monosodium phopshate 60.85 Sodium hemiphosphate 13.25

Panels of SAE 1010 cold roll steel were immersed in the above control bath at a temperature of 180 F. for a period of 60 seconds. After treatment, rinsing and drying the weight of coating deposited was estimated in accordance with the procedure published in Military Specification MIL-C-490, Cleaning and Phosphatizing of Ferrous and Zinc Coated Surfaces for Organic Protective Coatings, and the average weight of coating was found to be 75 mg./ sq. ft.

To this same control bath there was added tributyl phosphate in the amount of 1%, based upon the weight of the phosphatizing chemicals dissolved in the water of the phosphatizing solution. Additional panels were then processed under the same conditions of pH, temperature and time. These panels were examined for estimation of coating weight using the earlier referenced procedure, and the average coating weight was found to be 86 mg./sq. ft. This constitutes an increase in coating weight of 14%.

Further additions of tributyl phosphate were increment- 4 1y made to the bath up to a concentration of 1.25 based upon the weight of phosphatizing chemicals dissolved in water to constitute the bath. Additional panels processed showed coating weights averaging 104 mg./ sq. ft.

The 1.25% concentration proved to be the optimum amount of alkyl phosphoric acid ester. Further increases in tributyl phosphate content from 1.25% to saturation with an undissolved excess did not provide'higher coating weights. Conversely, the coating weights decreased progressively to a level of 36 mg./ sq. ft.

Similar tests performed with triethyl phosphate showed an increase in coating weight from 75 to 84 mg./sq. ft. with the addition of 1% triethyl phosphate. There was a further increase to 88 mg./sq. ft. at the 2% level of triethyl phosphate and a decline to mg./sq. ft. at the 3% level. The difference in behavior in comparison to tributyl phosphate is attributed to the moderate solubility of triethyl phosphate as compared to the butyl ester. It provided peak coatings at the 2% level whereas tributyl phosphate peaked at 1.25

Tricresyl phosphate, which shows the same order of water solubility as tributyl phosphate, gave peak coatings at the 1% level of concentration of 85 mg./sq. ft. and leveled off to 78 mg./sq. ft. at the 2 and 3% levels of concentration. The 2% addition gave a saturated solution of tricresyl phosphate with an undissolved excess.

EXAMPLE 2 Tests were performed with the phosphatizing compositions of Example 1, but applied by a spray process. Using a series of 50 panels of SAE 1010 cold roll steel, 25 were phosphatized with the control bath of Example 1 at 140 F. for 60 seconds by spray application, utilizing a gauge indicated discharge pressure of 20 p.s.i. The remaining 25 panelswere phosphatized with the control bath to which was added 1% tributyl phosphate, based upon the weight of phosphatizing chemicals present in the bath. A sampling of 3 panels from each set was made and coating weights were estimated yielding the following results:

Mg./sq. ft. Control bath 53 Control bath plus 1% tributyl phosphate 75 The remaining panels from both sets were then painted with a .25 mil of primer. They were scribed and subjected to accelerated salt spray tests in accordance with the procedures published by the American Society for Testing Materials for a period of 74 hours. At the end of this period the panels were removed from the salt spray cabinet, washed with plain water and examined for evidence of failure. In the controls of the area showed apparent failure mainly due to blistering of the paint. The test series showed only 12% area failure, with no failure on the scribe.

Humidity tests were simultaneously performed on certain of the painted panels under conditions of F. and 90% relative humidity. The results showed a similar order of failure for the controls and the test panels.

Unpainted panels from the control series and the test series were also subjected to humidity cabinet conditions with the following results:

Control panels50% of the area failed at 3 hours Test panels with tributyl phosphate50% of the area I failed in 7 hours EXAMPLE 3 Similar improvements are observed in the treatment of zinc coated steel as follows:

. V w sq. ft. Control bath panels 26 Control bath panels with tri-isopropyl phosphite 42 EXAMPLE 4 In certain paint preparation operations. the esters of acids of phosphorus can be incorporated in a cleaner bath which is employed prior to the phosphatizing stage with the result of improving the quality of coating and the protection afiorded the metals. Byway of example, hot roll sheets of SAE 1020steel are shot blasted to remove mill scale. The sheets-are then conveyed to a scrubber compartment which was charged with 0.5% of low foaming organic detergent such as a nonyl phenol polyoxyethylene ethanol (9 mols polyoxyethylene) dissolved in 180 F. water, and thence conveyed to a rollercoater device over which flowed a-phosphatizing bath having'the following compositions.

Material: Wt. percent Zinc dihydrogen phosphate 0.65 Phosphoric acid as H PO 1.40 Nitric acid as HNO -5. 0.12

Polystyrene sulfonic acid (200 molecular weight) 0.25 Ethylene glycol monobutyl ether 0.15 Water 97.43

Time f r 50% area tailuro,

Specimen hours Control panel (at 100 F. 90% relative humidity) 4 Panel cleaned with detergent containing tributyl phosphate (at 100 F. 90% relative humidity) 7 EXAMPLE 5 A conversion coating bath was made up at a concentration of 2 oz./ gallon, using the following compositions.

Material: Wt. percent Monosodium phosphate 98.75 Tributyl phosphate 1.25

Spray or immersion coating of cold r011 steel with the above bath produces a coating having an estimated weight of -20 mg./sq. ft.

EXAMPLE 6 The influence of esters of acids of phosphorus is also .6 pronounced in the phosphoric acid base type of compositions that are normally used at ambient temperature by a wipe-on, wipe-off method or immersion-rinse method for the removal of rust mill scale and oily soil to leave the ferrous surface in suitable condition for painting. A typical composition used for this purpose is as follows.

Material: Wt. percent Phosphoric acid, 75% 70 Ethylene glycol monobutyl ether 15 Water 15 Such a composition is normally diluted with 4 parts of water and applied wipe-on, wipe-off or used with wire brushing to remove heavy rust and scale or used as an immersion bath. Hot roll steel is etched with the solution to remove unwanted scale.

When 0.25% to 0.50%, based on the weight of the above composition,'of an ester of acid of phosphorus such as tributoxy ethyl phosphate is added, the scale is removed and a coating of iron phosphate is deposited. This coating affords protection againstafter rust which normally occurs when the ferrous part is rinsed after treatment.

EXAMPLE 7 A wear resistant and corrosion resistant converteddeposited coating can be imparted to steel with the following bath.

Material: Wt. percent Water 92.658 Manganese dihydrogen phosphate dihydrate 5.00 Orthophosphoric acid, 75% 2.00 Nitric acid, 60% 0.090 Nickel nitrate hexahydrate 0.002 Tri-2 ethyl hexyl phosphate 0.250

Parts were immersed in the above bath for a period of 20 to 45 minutes at a temperature of 180 to 200 F. The coating imparted was of the order of 1500 to 2000 mg./ sq. ft., with a fine grain and deep black in color.

EXAMPLE 8 Advantages are sometimes gained by embodying esters of acids of phosphorus in a cleaning composition containing suitable organic non-soap detergents (the use of which is well known to the art) to be used prior to phosphatizing or in admixture with the phosphatizing bath. A typical cleaning composition embodying an ester of an acid of phosphorus is shown as follows.

Material: Wt. percent Nonyl phenol polyoxyethylene ethanol (8-10 mols of oxyethylene) 7.5

Water 69.6

Tanic acid 0.5

Morpholene 0.7

Tributyl phosphate 1.7

Xylene sodium sulfonate 20.0

In the use of the above composition as a cleaner prior to phosphatizing it is used at a level of concentration of 1-2 qts./ gallons either by immersion or spray at a temperature in the range of l40180 F. Rinsing prior to entering the phosphatizing stage of the operation is not required.

When the cleaner is used in the phosphatizing bath, as, for example, the control bath of Example 1, it is used in the amount of 1-2 qts./100 gallons of bath in a temperature range of -180 F. The mixed bath can be used either by immersion or spray.

Typical, but not limiting examples, of esters of acids of phosphorus applicable for use in aqueous baths in accordance with the instant invention are tabulated as follows:

Triethyl phosphate Tripropyl phosphate Tributyl phosphate Tripentyl phosphate Trioctyl phosphate Tributoxyethyl phosphate Diparatertiary butyl phenyl monophenyl phosphate Triparatertiary butyl phenyl phosphate Tricresyl phosphate Triphenyl phosphate Sodium diphenyl phosphate Tributyl phosphite Tri-isopropyl phosphite Trilauryl phosphate Dilauryl phosphate As will be apparent from the above examples, the concentrations of the phosphorous acid esters may vary within relatively wide limits, from as low as about 0.1% of the ester, based on the weight of the phosphatizing chemical or chemicals in the bath, up to limit of a saturated solution of the ester in the bath.

Having fully described the present invention, it is to be understood that it is not to be limited to the specific details set forth, but is of the full scope of the appended claims.

I claim:

1. A phosphatizing composition for metals consisting essentially of an aqueous coating bath containing a watersoluble inorganic phosphorous component reactive with said metal to provide a corrosion-resistant coating thereon, said component selected from the group consisting of phosphoric acid and acid phosphate salts; and an ester of an acid of phosphorous, said ester having greater solvent power for acids of phosphorous than for salts of acids of phosphorous, the amount of said ester being minor based upon the amount of said phosphorous component but at least about 0.1% by weight based upon the weight of said phosphorous component.

2. The composition of claim 1 wherein said ester is a trialkyl phosphate.

3. The composition of claim 1 wherein said ester is present in an amount of at least about 1% by weight based upon said phosphorous component.

4. The composition of claim 1 wherein said ester is tripropyl phosphate.

5. The composition of claim 1 wherein said ester is tributyl phosphate.

6. The composition of claim 1 wherein said ester is tributoxy ethyl phosphate.

7. The composition of claim 1 wherein said inorganic phosphorous compound is manganese phosphate.

8. The composition of claim 1 wherein said inorganic phosphorous compound is zinc phosphate.

9. The composition of claim 1 wherein said inorganic phosphorous compound is iron phosphate.

10. The composition of claim 1 wherein said inorganic phosphorous compound is sodium phosphate.

11. A process for phosphatizing metals comprising applying to the metal surface a composition consisting essentially of an aqueous coating bath including a watersoluble inorganic phosphorous component reactive with said metal to provide a corrosion-resistant coating thereon, said component selected from the group consisting of phosphoric acid and acid phosphate salts; and an ester of an acid of phosphorous, said ester having greater solvent powerfor acids of phosphorous than for salts of acids of phosphorous, the amount of said ester being minor based upon the amount of said phosphorous component but at least about 0.1% by weight based upon the weight of said phosphorous component.

12. The process of claim 11 wherein said ester is a trialkyl phosphate.

13. The process of claim 11 wherein said ester is pres ent in an amount of at least about 1% by weight based upon said phosphorous component.

14. The process of claim 11 wherein said ester is tripropyl phosphate.

15. The process of claim 11 wherein said ester is tributyl phosphate.

16. The process of claim 11 wherein said ester is tributoxy ethyl phosphate.

17. The process of claim 11 wherein said inorganic phosphorous compound is manganese phosphate.

18. The process of claim 11 wherein said inorganic phosphorous compound is zinc phosphate.

19. The process of claim 11 wherein said inorganic phosphorous compound is iron phosphate.

20. The process of claim 11 wherein said inorganic phosphorous compound is sodium phosphate.

21. The process of claim 11 wherein an aqueous solution containing a minor amount of said ester is first applied to the metal surface and then an aqueous coating bath including said water-soluble phosphorous compound is applied to said surface.

22. The process of claim 11 wherein said metal is selected from the group consisting of ferrous alloys, zinc, aluminum and cadmium alloys.

References Cited UNITED STATES PATENTS 1,949,713 3/1934 Gravell 1486.15 2,126,181 8/1938 Strickland 1486.15 2,223,548 12/1940 Caplan 1486.15 2,224,695 12/ 1940 Prutton l48--6.15 2,408,155 9/1946 Thornbury 148-615 2,840,498 6/1958 Legue et a]. 148-615 2,873,196 2/1959 Baersky 148-6.15 X 2,891,909 6/1959 Hughes 260978 X 2,952,699 9/1960 Norman 148-615 3,247,027 4/1966 Fullhart et al. 148-6.15

RALPH S. KENDALL, Primary Examiner. 

